x86/entry/64: Stop initializing TSS.sp0 at boot

In my quest to get rid of thread_struct::sp0, I want to clean up or
remove all of its readers.  Two of them are in cpu_init() (32-bit and
64-bit), and they aren't needed.  This is because we never enter
userspace at all on the threads that CPUs are initialized in.

Poison the initial TSS.sp0 and stop initializing it on CPU init.

The comment text mostly comes from Dave Hansen.  Thanks!

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/ee4a00540ad28c6cff475fbcc7769a4460acc861.1509609304.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4e7fb9c3bfa5c170015ac5e67ba5a55f776dd3e8..cdf79ab628c244288a01561702e9c93d2d9ab348 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1570,9 +1570,13 @@ void cpu_init(void)
        initialize_tlbstate_and_flush();
        enter_lazy_tlb(&init_mm, me);
 
-       load_sp0(current->thread.sp0);
+       /*
+        * Initialize the TSS.  Don't bother initializing sp0, as the initial
+        * task never enters user mode.
+        */
        set_tss_desc(cpu, t);
        load_TR_desc();
+
        load_mm_ldt(&init_mm);
 
        clear_all_debug_regs();
@@ -1594,7 +1598,6 @@ void cpu_init(void)
        int cpu = smp_processor_id();
        struct task_struct *curr = current;
        struct tss_struct *t = &per_cpu(cpu_tss, cpu);
-       struct thread_struct *thread = &curr->thread;
 
        wait_for_master_cpu(cpu);
 
@@ -1625,9 +1628,13 @@ void cpu_init(void)
        initialize_tlbstate_and_flush();
        enter_lazy_tlb(&init_mm, curr);
 
-       load_sp0(thread->sp0);
+       /*
+        * Initialize the TSS.  Don't bother initializing sp0, as the initial
+        * task never enters user mode.
+        */
        set_tss_desc(cpu, t);
        load_TR_desc();
+
        load_mm_ldt(&init_mm);
 
        t->x86_tss.io_bitmap_base = offsetof(struct tss_struct, io_bitmap);

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index bd6b85fac66696da70e316656ad6f0d51291f8aa..ff8a9acbcf8bddc4eba6b18193ed622e5a01ef04 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -48,7 +48,13 @@
  */
 __visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = {
        .x86_tss = {
-               .sp0 = TOP_OF_INIT_STACK,
+               /*
+                * .sp0 is only used when entering ring 0 from a lower
+                * privilege level.  Since the init task never runs anything
+                * but ring 0 code, there is no need for a valid value here.
+                * Poison it.
+                */
+               .sp0 = (1UL << (BITS_PER_LONG-1)) + 1,
 #ifdef CONFIG_X86_32
                .ss0 = __KERNEL_DS,
                .ss1 = __KERNEL_CS,